This invention relates to RF admittance measuring systems for monitoring the condition of materials, and more particularly to such systems which are adapted to provide an indication of the level of materials.
In the past, so-called two-wire transmitters have been utilized to monitor the condition of materials at a remote location. Typicaly, a two-wire transmitter disposed at a remote location is connected in series relationship with a local transmitter comprising a power supply and a load through a pair of transmission wires. As the condition being monitored at the local transmitter varies, the effective series resistance across the remote transmitter varies so as to produce a change in the current flowing through the two-wire circuit. The level of the current represents, in many instances being proportional to, the condition being monitored. One of the advantages of a system of the type under discussion is the low power consumption, since the amount of power which is available to the local transmitters may be limited. In addition, certain applications require that the local transmitters be "intrinsically safe" so as to permit its use in monitoring conditions in an explosive environment. Under such circumstances, the low energy level which is associated with low power consumption becomes important in precluding the possibility of ignition and explosion.
Although the state of the art in two-wire transmitters is adequate for monitoring many types of conditions, nonetheless there are deficiencies in prior art RF admittance measuring systems. In particular, it will be recognized that the electrical parameters of the material to be measured are intrinsically part of the measurement system with RF measurement devices. Hence, different types of materials will provide different signal characteristics, requiring recalibration of the transmitting apparatus when different materials are to be measured. In any environment where the material remains the same, only its level varying, an initial calibration of the system should suffice. However, in some applications the composition of material whose level is to be monitored varies from time to time, necessitating calibration or readjustment of the system each time the composition varies.
According to copending application Ser. No. 963,300 filed Nov. 24, 1978, assigned to the assignee of this invention, a probe may be provided which embodies a pair of sensing elements; one for sensing the level of material in a vessel, and a second for sensing the composition thereof. In each case, the sensing element produces a change in the apparent admittance of the probe and an admittance-related signal is derived for producing an indication of both the level and the composition of the material. Once such signals are derived it is possible to use the composition-related signal to modify or compensate the level-related signal, so as to provide a signal reflecting the compensated level of the material despite changes in its composition and/or insulation from time to time.
Certain problems inhere in the above approach. For instance, both the composition and the level-related admittance signal must be detected, processed and combined, and to the extent that two separate two-wire transmitters are required, difficulties may be encountered in maintaining the amplitude and phasing of the transmitters within sufficiently close tolerance to permit accurate and low cost measuring.